Noise canceling apparatus for a power converter

ABSTRACT

A noise canceling apparatus is provided that uses magnetically-coupled windings to cancel noise currents or noise voltages from a power converter. The apparatus may include a series voltage source or a shunt current source that is placed at input or output terminals of a power converter to eliminate the noise generated from the power converter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to the field of power converters. Morespecifically, the invention relates to the field of reducing noise inpower converters such as AC to DC converters, DC to AC converters or DCto DC converters.

2. Background of the Invention

A modern switching-mode power converter is light weight and provideshigh efficiency power conversion. One of the problems with this type ofconverter, however, is that it generates undesirable switching noise.This switching noise manifests itself as a ripple voltage or ripplecurrent that is generated by the switching-mode at the input or outputside of the converter. Reduction of such ripple voltage or currentbecomes a necessary design requirement in order to comply withinternational standards. Some prior art provides a means to reduce suchripple current. For example, a traditional means to suppress ripplevoltage or current is by implementing a passive filter. As shown in FIG.1, a prior art converter implements a simple LC circuit. A pair ofinductors, L₁ and L₂ are coupled to a pair of capacitors C₁ and C₂.Together, these components are coupled to a converter that is modeled asa diode D₁, a capacitor C₃, a resistor R₁ and a transistor M₁ that actsas a switch for the converter. A passive filter, however, requires bulkycomponents that take up the limited space within the power converter.Other prior art work implements active cancellation techniques to reducethe ripple noise. For example, in U.S. Pat. No. 4,274,133 to Cuk aconverter is disclosed that cancels ripple current by matching acoupling coefficient of two inductors within the converter. In U.S. Pat.No. 5,038,263 to Marrero a circuit is disclosed with a winding coupledto the main transformer of the converter for diverting ripple current toa capacitor. The coupling ratio of the transformer windings reduces theac ripple current input switching. In Marrero, an inductor is coupled inseries with the input to provide constant current. This inductor,however, is bulky and adds an extra component in the power conversionpath. In U.S. Pat No. 6,008,999 also to Marrero a converter is disclosedhaving an additional winding that effects the output inductance of theconverter. The coupling ratio of the additional winding and outputwinding reduces input and output switching ripple.

These prior art methods have drawbacks that limit the utility of thepower converter. For example, traditional passive filters require bulkyinductors and capacitors that increase component count and spacerequirements. Known active noise cancellation techniques reducecomponent size and count, but require careful magnetic coupling betweeneach winding in the main magnetic component. The cancellation effect isnot achieved in these circuits without a tightly-coupled magnetic field.The main magnetic component must be made precisely to satisfy both powerconversion and noise cancellation requirements. This becomes an addedconstraint to the design and increases the difficulty to manufacture theconverter.

SUMMARY OF THE INVENTION

The present invention provides a general solution for canceling ripplecurrent generated by a power converter. In general, two methods areprovided, a series voltage source or a shunt current source, which areplaced at the input terminals of a power converter to eliminate ripplecurrent generated by the converter. This noise cancellation apparatuscan be applied to any power converter because it is a separate unit thatcaptures the noise signal and generates a cancellation signal equal inmagnitude, but opposite in phase to the noise signal, in order to reducethe undesirable noise, such as switching voltage or ripple current of aswitching-mode power supply.

According to one aspect of the invention, an AC voltage can be insertedin series with the input terminal of the power converter to reduce thenoise voltage. The AC voltage is equal in magnitude but opposite inphase to the noise voltage generated by an AC noise current, or theequivalent of an AC noise current, coupled to the input impedance of thepower converter. The noise voltage at the input is thus cancelled.

According to another aspect of the invention, a shunt AC current sourcecan be inserted in parallel with the input terminal of a powerconverter. If the shunt AC current has equal magnitude, but oppositephase, of the noise current generated by the power converter, then theAC current flow into the input terminal is cancelled.

According to yet another aspect of the invention, an AC voltage can beinserted in series with the input terminal of a power converter. Thisvoltage source can be derived from a noisy voltage node or itsequivalent within the converter. Voltage scaling is provided byimpedance networks and this voltage source can cancel out noise at theconverter input.

The design of these cancellation circuits, unlike traditional filterdesigns, is not dependent on converter input source impedance. The noisecancellation can also be incorporated with a traditional filter tofurther reduce the noise level associated with the ripple current andvoltage.

Accordingly, it is an object of the present invention to provide noisecancellation circuits for power converters. The noise cancellationcircuits can be placed on the input terminals or output terminals of thepower converter. The noise reduction apparatus can reduce the noise ofdifferent topologies of power converters. Furthermore, the noisereduction apparatus has a low component count and reduces the size ofthe components in the design.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and from the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(Prior Art) is a simplified equivalent circuit of a boostconverter with a filtering circuit at the input terminal;

FIG. 2A is a schematic diagram of a circuit showing a series voltagenoise canceling source controlled by converter noise current;

FIG. 2B is a schematic diagram of a circuit showing a parallel noisecanceling source controlled by converter noise current;

FIG. 2C is a schematic diagram of a circuit showing a parallel noisecanceling source controlled by converter noise voltage;

FIG. 2D is a schematic diagram of a circuit showing a series voltagenoise canceling source controlled by converter noise voltage;

FIG. 3 is a preferred embodiment of the present invention using a seriesvoltage source to reduce noise created by ripple;

FIG. 4 is an example circuit of the preferred embodiment of FIG. 3coupled to a boost converter;

FIG. 5A is an example input noise spectrum of a boost converter withoutnoise cancellation;

FIG. 5B is an example input noise spectrum of the boost converter withnoise cancellation of FIG. 4;

FIG. 6A is a plot showing the measured input ripple current of the boostconverter without noise cancellation in the time domain and in thefrequency domain;

FIG. 6B is a plot showing the measured input ripple current of the boostconverter with noise cancellation in the time domain and in thefrequency domain;

FIG. 7 is another preferred embodiment of the present invention couplinga parallel current source derived from the noise voltage to a converterin order to reduce ripple;

FIG. 8 is an example circuit of the embodiment of FIG. 7 coupled to aboost converter;

FIG. 9 is another preferred embodiment of the present invention couplinga series voltage source derived from the noise voltage to a converter inorder to reduce ripple; and

FIG. 10 is an example circuit of the embodiment of FIG. 9 coupled to aboost converter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, noise detected at the input of a switching power converteris generated by a pulsating current or a pulsating voltage within theconverter. Each of these noise sources is present in switching powerconverters, but which noise source dominates the noise signal depends onthe converter. If the converter includes high pulsating current, thenthe pulsating current model may be the most appropriate model for noisereduction. If the converter includes a high pulsating voltage, however,then the pulsating voltage model may be the most appropriate model fornoise reduction. These models form the basis for the generation ofcancellation signals to eliminate noise at the input.

FIG. 2A is a schematic diagram of a circuit showing a series voltagenoise-canceling source controlled by converter noise current. Thecurrent-based noise source is modelled by a switching current sourceI₁₁. An input capacitor C₁₂ is in parallel to the input and functions asan energy storage element. A series impedance of the input, Z₁₃, is themodeled impedance of the converter. In order to eliminate noise at Z₁₃,an active voltage source, V₁₄, is put in series with the input. Thisvoltage source is matched to the current change such that the voltagesource changes oppositely to the ripple voltage across capacitor C₁₂,thus canceling the AC voltage that would have appeared across Z₁₃ andeliminating input noise. A direct method for matching the voltage sourceto the noise is to couple the current change at I₁₁ to the seriesvoltage source V₁₄.

FIG. 2B is a schematic diagram of a circuit showing a parallel noisecanceling source controlled by converter noise current. A switchingsource I₂₁ represents a current-based noise source. An input capacitorC₂₂ is in parallel to the input and functions as an energy storageelement. A series impedance of the input, Z₂₄, is the modeled impedanceof the converter. In order to eliminate noise at Z_(24,) an activecurrent source, I₂₃, is coupled in parallel to the input. This currentsource produces a current opposite to that of the noise source I₂₁ sothat the input impedance Z₂₄ does not see any ripple current.

FIG. 2C is a schematic diagram of a circuit showing a parallel noisecanceling source controlled by converter noise voltage. A voltage basednoise source is represented by pulsating voltage source V₃₁ in serieswith an impedance Z₃₂. A capacitor C₃₃ is parallel to the input. Aseries impedance of the input, Z₃₅, is the modeled impedance of theconverter. In order to eliminate noise at Z₃₅, an active current sourceI₃₄ is coupled in parallel to the input. The current source I₃₄ producesa current opposite to that drawn by the noise source V₃₁. The impedanceZ₃₅ does not see any ripple current. A method for matching the voltagenoise source to the current source I₃₄ is to directly couple the currentsource I₃₄ to the noise source V₃₁.

FIG. 2D is a schematic diagram of a circuit showing a series voltagenoise canceling source controlled by converter noise voltage. A voltagebased noise source is represented by pulsating voltage source V₄₁ inseries with an impedance Z₄₂. A capacitor C₄₃ is parallel to the input.A series impedance of the input, Z₃₅, is the modeled impedance of theconverter. In order to eliminate noise at Z₃₅, an active voltage sourceV₄₄ is coupled in series to the impedance Z₄₅. This voltage source ismatched to the current change such that the voltage source changesoppositely to the ripple voltage across capacitor C₄₃, thus cancelingthe AC voltage that would have appeared across Z₄₅ and eliminating inputnoise. The voltage source V₄₄ produces a voltage opposite to that of thenoise source V₄₁ so that the input impedance Z₂₄ does not see any ripplevoltage.

The schematic diagrams of FIGS. 2A through 2D represent the idealizedcontrol mechanism for reducing ripple noise by using either a shuntcurrent (FIGS. 2B and 2C) or by using a voltage source (FIGS. 2A and2D). The use of the shunt current or voltage source as a controller isapplicable to a circuit where the ripple noise is modeled as a currentnoise source (FIGS. 2A and 2B) or as a voltage noise source (FIGS. 2Cand 2D).

Turning now to FIG. 3, a preferred embodiment of the present inventionis shown in which a series voltage source is used to reduce noisecreated by ripple. This noise canceling apparatus is an embodiment ofthe idealized voltage source/ripple current configuration of FIG. 2A.The apparatus includes a transformer T₅₄ that is coupled to a pair ofwindings, W₅₂ and W₅₃. The winding W₅₃ is in parallel to an impedanceZ₅₁. Both the W₅₃ winding and the Z₅₁ impedance are in series with aripple noise current source I₅₁. An input impedance Z₄₅ is in serieswith the W₅₂ winding and an energy storing capacitor C₅₀. The energystoring capacitor C₅₀ is, in turn, in series with the W₅₃ winding andthe current source I₅₁.

A cancellation voltage is generated by detecting the voltage drop acrossthe impedance Z₅₁ measured across the winding W₅₃ caused by theswitching ripple current I₅₁. The cancellation voltage across W₅₂ isdetermined by the transformer ratio of T₅₄ and the impedance Z₅₁,because the turn ratio of W₅₂ and W₅₃ is unity.

The following equations show the mathematical relationship betweenvoltage drops across components in the circuit,

 v(C ₅₀)=i(I ₅₁).Z(C ₅₀)  (1)

v(C ₅₀)=v(W ₅₂)  (2)

v(W ₅₂)=i(I ₅₁).(Z ₅₁ //Z _(LT1))  (3)

where, Z_(LT1) is the input impedance of the transformer T₅₄ at windingW₅₃.

Solving the above equations, Z₅₁ can be found for zero input noise as:$\begin{matrix}{Z_{51} = \frac{{Z\left( C_{50} \right)}Z_{LT1}}{Z_{LT1} - {Z\left( C_{50} \right)}}} & (4)\end{matrix}$

Assuming Z_(LT1)>>Z(C₅₀), hence Z₅₁ can be approximated as,

Z ₅₁ =Z(C ₅₀),  (5)

The above assumption and the result shows that a component that has animpedance equal to that of the primary filter capacitor (C₅₀) andcoupled in parallel to winding W₅₃ of transformer T₅₄ can providecancellation of the noise voltage. If the components of the filterinclude components other than a capacitor, the impedance Z₅₁ is modeledas the total impedance of the primary filter components.

If Z₅₁ is equal to Z(C₅₀), then the voltage drop on both impedances areequal because the parallel impedance of T₅₄ is large. Transformer T₅₄thus provides an equal amplitude, but opposite-phase, waveform acrossthe W₅₂ winding to cancel out the voltage change across C₅₀ due to thenoise current source I₅₁.

In a more general case where the turn ratio of W₅₂ and W₅₃ is not equalto one, the condition for noise cancellation is $\begin{matrix}{\frac{Z_{51}}{Z\left( C_{50} \right)} = \frac{N\left( W_{53} \right)}{N\left( W_{52} \right)}} & (6)\end{matrix}$

where N(W₅₃) is the number of turns of winding W₅₃ and N(W₅₂) is thenumber of turns of winding W₅₂.

FIG. 4 is an example circuit of the preferred embodiment of FIG. 3coupled to a boost converter. The noise source I₅₁ in FIG. 3 is replacedby the boost converter. The boost converter is modeled as a 100 uHinductor in series with an input transistor and in parallel with a 10 uFcapacitor and a 100 Ω resistor through a diode D₄₂. An input waveformhaving a rise time of 0.1 us, a fall time of 0.1 us, a peak time of 5 usand a period of 10 us is input into the circuit through the gate of thetransistor. The capacitor C₅₀ is a 1 uF capacitor that is in series witha 0.05 Ω resistor and a 0.1 uH inductor. Each winding W₅₂ and W₅₃ is a20 uH winding. The impedance placed in parallel to the 20 uH winding W₅₃must then be equal to the combined impedance of the 1 uF capacitor, the0.050Ω resistor and the 0.1 uH inductor. The transformer T₅₄ thengenerates a voltage drop in the W₅₂ winding that is equal in magnitudeto the sensed voltage drop across the W₅₃ winding, but the voltage dropis opposite in phase.

FIG. 5A is an example input noise spectrum of a boost converter withoutnoise cancellation. The input current spectrum was calculated withoutthe coupling transformer T₅₄, but with the 20 uH winding W₅₃ placed inseries with the boost converter. The first harmonic of the noise signalcontributes most of the amplitude to the noise spectrum.

FIG. 5B is an example input noise spectrum of a boost converter withnoise cancellation as shown in FIG. 4. The boost converter with noisecancellation has a noise spectrum that is dramatically attenuated byusing a small size 20 uH 1:1 cancellation transformer T₅₄ with aparallel low voltage 1 uF capacitor.

FIG. 6A is a plot showing the measured input ripple current of the boostconverter without noise cancellation in the time domain (upper plot) andfrequency domain (lower plot). FIG. 6B is a plot showing the measuredinput ripple current of the boost converter with noise cancellation inthe time domain (upper plot) and frequency domain (lower plot). FIG. 6Ashows that a first harmonic of the switching ripple current has a largemagnitude and the accompanying sinusoidal wave in the time domain isrelated to the large magnitude of the first harmonic. After adding thecancellation circuit to the converter, the first harmonic of theswitching ripple current is reduced, as shown in FIG. 6B. The reductionin the frequency domain corresponds to the relatively flat waveformsignal in the time domain.

Turning now to FIG. 7, another preferred embodiment of the presentinvention couples a parallel current source derived from the noisevoltage to a converter in order to reduce ripple. This embodimentcorresponds to the idealized noise cancellation method shown in FIG. 2C.In this circuit, a noise voltage V₆₁ produces noise at the input. Animpedance Z₂₀ is in series with the noise voltage V₆₁. A capacitor C₆₃and a winding W₆₄ are in parallel with the noise voltage. A transformerT₆₈ couples the winding W₆₄ to a winding W₆₅. An impedance Z₆₆ is placedin series with the W₆₅ winding. The windings W₆₄ and W₆₅ are bothparallel to an input impedance Z₆₇.

The transformer T₆₈ detects the noise voltage V₆₁ and converts it to ashunt current source. The transformer T₆₈ accomplishes these functionswith the windings W₆₄ and W₆₅. Winding W₆₄ captures the noise voltageand winding W₆₅ produces the compensation current. The capacitor C₆₃ isplaced in series with winding W₆₄ so that it picks up AC noise only anddoes not interfere with normal converter operation. The impedance Z₆₆ isplaced in series with winding W₆₅ in order to produce a correspondingnoise canceling current.

When a noise current is generated by the noisy voltage source V₆₁through the impedance Z₂₀, then the impedance Z₆₆ drives a correspondingcurrent which cancels the noisy current. Assuming the magnetizingimpedance of sensing winding W₆₄ is high, the condition for null noisecurrent flow at the input impedance Z₆₇ can be approximately related as$\begin{matrix}{\frac{Z_{66}}{Z_{20}} = \frac{N\left( W_{65} \right)}{N\left( W_{64} \right)}} & (7)\end{matrix}$

where N(W₆₅) is the number of turns of winding W₆₅ and N(W₆₄) is thenumber of turns of winding W₆₄.

Impedance networks are needed in this circuit, but they may notnecessarily require extra components, because the impedances may becomponents of the corresponding switching converter. For example Z₂₀ maybe the input or output inductor of a boost or buck converter,respectively.

Turning now to FIG. 8, an example circuit of the embodiment of FIG. 7coupled to a boost converter is set forth. The circuit is applied to aboost converter comprising an inductor L₇₅, transistor M₇₇, diode D₇₈,resistor R_(load) and output capacitor C₇₉. The transistor M₇₇ iscontrolled through a source 76.

The noise cancellation circuit comprises a transformer T₇₀ which has twowindings W₇₂ and W₇₃. A sensing winding W₇₂ detects noise voltage acrossthe switch M₇₇ and transfers the noise signal to a compensating windingW₇₃. A capacitor C₇₁ is coupled in series with an inductor L₇₄. Aninductor L₇₅, which may be the input inductor of the boost converter, isin series with the components of the boost converter. The inductor L₇₄and the capacitor C₇₁ are an impedance network to match the impedance ofthe input inductor L₇₅ and the output capacitor C₇₉. The impedancenetwork translates the noise voltage signal derived from the sensingwinding W₇₂ and passed to the compensating winding W₇₃ into an equalmagnitude, opposite phase current signal. The current signal thuscancels out the noise at the input.

FIG. 9 is another preferred embodiment of the present invention couplinga series voltage source derived from the noise voltage to a converter inorder to reduce ripple. It consists of a noise voltage source V₈₁ whichis connected in series with an impedance Z₈₂. An input impedance of theconverter is modeled as Z₈₇ and a capacitor C₈₅. The cancellationcircuit consists of a transformer T₈₆ coupled to a pair of windings W₈₄and W₈₅. The sensing winding W₈₄ is in series with a capacitor C₈₃ andan impedance Z₈₀. An impedance network Z₈₈ is parallel to the sensingwinding W₈₄. The compensating winding W₈₅ is in series with theimpedance Z₈₂ and the noise voltage source V₈₁.

In this circuit, the transformer T₈₆ detects the noise voltage V₈₁, andconverts it to a series voltage source. The sensing winding W₈₄ capturesthe noise voltage. The compensating winding W₈₅ produces a compensatingvoltage in series with the noise voltage V₈₁. The capacitor C₈₃ isplaced in series with winding W₈₄ so that it picks up AC noise only anddoes not interfere with normal converter operation. The impedancenetwork Z₈ and the impedance Z₈₀ adjust the noise cancellationcharacteristics of the circuit. Winding W₈₅ is placed in series with theinput so that it produces a voltage signal equal in magnitude butopposite in phase to cancel the noise voltage.

FIG. 10 is an example circuit of the embodiment of FIG. 9 coupled to aboost converter. The boost converter is modeled as a circuit andcomprises an inductor L₉₆, a transistor M₉₈, a diode D₉₇, a resistorR_(load) and an output capacitor C₉₉. A source 100 controls theswitching of the transistor M₉₈.

The cancellation circuit is modeled as a pair of windings W₉₂ and W₉₃coupled by a transformer T₉₁. The sensing winding W₉₂ is in parallelwith an impedance network Z₉₄ and in series with a capacitor C₉₅ and animpedance Z₉₀.

Winding W₉₂ detects noise voltage across the switch M₉₈ and transfersthe noise signal to winding W₉₃. The impedance network Z_(94,) theimpedance Z₉₀, and the capacitor C₉₅ are tuned to couple the noisecanceling voltage across the winding W₉₃ to the voltage noise controlledthrough the switch M₉₈. These components are modeled to match theimpedance of the booster converter components L₉₆ and C₉₉. The voltagesignal generated across the W₉₃ winding is thus equal in magnitude butopposite in phase to the voltage drop generated across the transistorM₉₈.

Each of the three embodiments of the cancellation circuit describedabove can be applied to the input or output of a power converter.Coupling the cancellation circuit to the output cancels output ripplewhile coupling the cancellation circuit to the input cancels inputripple. The configuration and operating principles are the same for bothinput cancellation and output cancellation.

The preferred embodiments described with reference to the attacheddrawing figures are presented only to demonstrate certain examples ofthe invention. Other elements, steps, methods, and techniques that areinsubstantially different from those described above and/or in theappended claims are also intended to be within the scope of theinvention.

What is claimed is:
 1. A noise canceling circuit for use with a powerconverter that generates a noise current, comprising: a transformerhaving a first winding and a second winding that are magneticallycoupled and are configured in series to the power converter; animpedance network coupled in parallel to the first winding; and a filternetwork coupled in series with the second winding and in parallel withthe power converter, wherein the impedance of the impedance network andthe filter network are approximately equivalent; wherein the noisecurrent is coupled to the first winding of the transformer, whichmeasures a corresponding voltage drop across the first impedance networkand causes the second winding to generate a noise cancellation voltagethat compensates for the noise current generated by the power converter.2. The noise canceling circuit of claim 1, wherein the filter networkcomprises a single capacitor.
 3. The noise canceling circuit of claim 1,wherein the filter network comprises a capacitor coupled in series witha resistor.
 4. The noise canceling circuit of claim 3, wherein thefilter network further comprises an inductor.
 5. The noise cancelingcircuit of claim 1, wherein the power converter includes an input portfor receiving input power and an output port for providing convertedpower.
 6. The noise canceling circuit of claim 5, wherein the noisecanceling circuit is coupled to the input port of the power converter.7. The noise canceling circuit of claim 5, wherein the noise cancelingcircuit is coupled to the output port of the power converter.
 8. Thenoise canceling circuit of claim 5, wherein the power converter iscoupled to an input power source, and wherein the noise cancelingcircuit is coupled between the input power source and the input port ofthe power converter.
 9. The noise canceling circuit of claim 1, whereinthe inductance of the first winding and the second winding areapproximately equivalent.
 10. The noise canceling circuit of claim 1,wherein the noise cancellation voltage is an AC noise cancellationvoltage.
 11. The noise canceling circuit of claim 10, wherein the ACnoise cancellation voltage is equal in magnitude but opposite in phaseto the corresponding voltage drop across the first impedance network.12. The noise canceling circuit of claim 1, wherein the cancellationvoltage is equal in magnitude but opposite in phase to the correspondingvoltage drop across the first impedance network.
 13. The noise cancelingcircuit of claim 1, wherein the power converter is a boost converter.14. The noise canceling circuit of claim 1, wherein the power converteris a buck converter.
 15. The noise canceling circuit of claim 1, whereinthe power converter is a switching power converter.
 16. The noisecanceling circuit of claim 1, wherein the impedance of the noisecanceling circuit is substantially less than the input impedance of thepower converter.
 17. The noise canceling circuit of claim 1, wherein theturn ratio of the first and second windings of the transformer is unity.18. A noise canceling circuit for use with a power converter thatgenerates a noise voltage, comprising: a transformer having a firstwinding and a second winding that are magnetically coupled and areconfigured in parallel to each other, wherein the first winding iscoupled in parallel to the power converter and the second winding iscoupled to an internal node of the switching power converter where thenoise voltage is generated; and an impedance network coupled in serieswith the first winding; wherein the second winding of the transformermeasures the noise voltage at the internal node and causes the firstwinding to generate a noise cancellation current that compensates forthe noise voltage generated by the power converter.
 19. The noisecanceling circuit of claim 18, wherein the impedance network comprises asingle capacitor.
 20. The noise canceling circuit of claim 18, whereinthe impedance network comprises a capacitor in series with an inductor.21. The noise canceling circuit of claim 18, wherein the power converterincludes an input port for receiving input power and an output port forproviding converted power.
 22. The noise canceling circuit of claim 21,wherein the noise canceling circuit is coupled to the input port of thepower converter.
 23. The noise canceling circuit of claim 21, whereinthe noise canceling circuit is coupled to the output port of the powerconverter.
 24. The noise canceling circuit of claim 21, wherein thepower converter is coupled to an input power source, and wherein thenoise canceling circuit is coupled between the input power source andthe input port of the power converter.
 25. The noise canceling circuitof claim 18, wherein the noise cancellation current is an AC noisecancellation current.
 26. The noise canceling circuit of claim 18,wherein a noise cancellation voltage is generated across the impedancenetwork by the noise cancellation current, and wherein the noisecancellation voltage is equal in magnitude but opposite in phase withthe noise voltage.
 27. The noise canceling circuit of claim 18, whereinthe power converter is a switching converter.
 28. The noise cancelingcircuit of claim 27, wherein the switching converter is a boostconverter.
 29. The noise canceling circuit of claim 28, wherein theboost converter includes an input port, an output port, an inductorcoupled in series with the input port, a capacitor coupled in parallelwith the output port, a diode coupling the input port to the outputport, and a switching transistor coupled in parallel to the output port.30. The noise canceling circuit of claim 29, wherein the internal nodeis connected to the output of the switching transistor.
 31. The noisecanceling circuit of claim 29, wherein the impedance of the impedancenetwork is approximately equivalent to the impedance of the inductor andthe capacitor of the switching power converter.
 32. A noise cancelingcircuit for use with a power converter that generates a noise voltage,comprising: a transformer having a first winding and a second windingthat are magnetically coupled, wherein the first winding is coupled inseries with the power converter and the second winding is coupled to aninternal node of the power converter where the noise voltage isgenerated; and an impedance network coupled to the second winding;wherein the second winding of the transformer measures the noise voltageat the internal node and causes the first winding to generate a noisecancellation voltage that compensates for the noise voltage generated bythe power converter.
 33. The noise canceling circuit of claim 32,wherein the impedance network comprises a single capacitor coupled inseries with the second winding.
 34. The noise canceling circuit of claim32, wherein the power converter includes an input port for receivinginput power and an output port for providing converted power.
 35. Thenoise canceling circuit of claim 34, wherein the noise canceling circuitis coupled to the input port of the power converter.
 36. The noisecanceling circuit of claim 34, wherein the noise canceling circuit iscoupled to the output port of the power converter.
 37. The noisecanceling circuit of claim 34, wherein the power converter is coupled toan input power source, and wherein the noise canceling circuit iscoupled between the input power source and the input port of the powerconverter.
 38. The noise canceling circuit of claim 32, wherein thenoise cancellation voltage is an AC noise cancellation voltage.
 39. Thenoise canceling circuit of claim 32, wherein a noise cancellationvoltage is equal in magnitude but opposite in phase with the noisevoltage.
 40. The noise canceling circuit of claim 32, wherein the powerconverter is a switching converter.
 41. The noise canceling circuit ofclaim 40, wherein the switching converter is a boost converter.
 42. Thenoise canceling circuit of claim 41, wherein the boost converterincludes an input port, an output port, an inductor coupled in serieswith the input port, a capacitor coupled in parallel with the outputport, a diode coupling the input port to the output port, and aswitching transistor coupled in parallel to the output port.
 43. Thenoise canceling circuit of claim 22, wherein the internal node isconnected to the output of the switching transistor.
 44. The noisecanceling circuit of claim 43, wherein the impedance of the impedancenetwork is approximately equivalent to the impedance of the inductor andthe capacitor of the switching power converter.
 45. A noise cancelingcircuit for use with a power converter that generates a noise current,comprising: measurement circuitry coupled to the power converter formeasuring the noise current by generating a noise measurement voltagethat corresponds to the noise current; and compensation circuitrycoupled to the measurement circuitry and the power converter forgenerating a noise cancellation voltage in series with the powerconverter, wherein the noise cancellation voltage is equal in magnitudebut opposite in phase with the noise measurement voltage.
 46. The noisecanceling circuit of claim 45, wherein the measurement circuitry iscoupled in series with the power converter and includes a first windingand an impedance network coupled in parallel, wherein the noise currentis coupled to the measurement circuitry and passes through the impedancenetwork, thus causing the first winding to measure the noisecancellation voltage.
 47. The noise canceling circuit of claim 46,wherein the compensation circuitry includes a second windingmagnetically coupled to the first winding and a filter network coupledin series with the second winding and in parallel with the powerconverter, wherein the impedance of the filter network and the impedancenetwork are approximately equivalent.
 48. A noise canceling circuit foruse with a power converter that generates a noise voltage, comprising:measurement circuitry coupled to the power converter for measuring thenoise voltage; and compensation circuitry coupled to the measurementcircuitry and the power converter for generating a noise cancellationvoltage in series with the power converter, wherein the noisecancellation voltage is equal in magnitude but opposite in phase withthe noise voltage.